Hybrid relays of this kind have been used for controlling the supply of electric current to inductive loads such as AC motors for the purpose of suppressing the generation of electric arcs when starting and stopping the supply of electric power to the loads. An example of such a hybrid relay is shown in FIG. 5.
The hybrid relay 101 is interfaced between an input signal circuit 102 and an output load circuit 103. The input signal circuit 102 comprises a control power source 104 and a drive switch 105 for producing an input signal for the hybrid relay 101. The output load circuit 120 comprises a load 110 such as an AC motor, and a load power source 111 such as an AC power source which is connected in series with the load 110. In the input end of the hybrid relay 101 are arranged a relay coil 106, a timer circuit 107 consisting of a resistor 121 and a capacitor 116 which are connected in series with one another, and a light emitting element 109 which forms a part of a photo-triac (triode alternating-current switch) 108, in mutually parallel relationship. In the output end of the hybrid relay 101 ar arranged a light receiving element 112 and a triac 113 which jointly form another part of the phototriac 108, an absorber circuit 114 for eliminating spurious pulse signals, and an output contact mechanism 115, and these circuit elements are connected across the load 110 and the load power source 111 in mutually parallel relationship.
When the switch 105 is turned on, the relay coil 106 is energized on the one hand and the light emitting element 109 receives a supply of electric power on the other hand. The energization of the relay coil 106 causes a conductive state of the output contact mechanism 115 and the light emitted from the light emitting element 109 brings the light receiving element 112 into conductive state with the result that the triac 113 turns into conductive state by receiving a voltage at its gate. However, since the photo-triac 108 operates electrically whereas the output contact mechanism 115 operates mechanically, the triac 113 turns into conductive state substantially before the output contact mechanism 115 does, and the output contact mechanism 115 therefore becomes conductive only after the output load circuit 103 has turned into conductive state due to the conduction of the triac 113, whereby the generation of electric arcs in the output contact mechanism 115 is avoided.
Conversely, when the switch 105 is turned off, the light emitting element 109 continues to emit light before the capacitor 116 of the timer circuit 107 is electrically discharged to a sufficient extent, so that the output contact mechanism 115 is disconnected while the triac 113 is still in conductive state. Thus, the triac 113 is brought into non-conductive state only after the output contact mechanism 115 is brought into non-conductive state, the generation of electric arcs in the load contact mechanism 115 is again avoided.
By preventing the generation of electric arcs in the output contact mechanism 115 as described above, the wear of the contact points is reduced and their durability is improved.
However, conventional hybrid relays have the following problems. First of all, when the switch 105 is turned on, the triac 113 becomes conductive before the output contact mechanism 115 does under normal condition, but, if the resistive value of a resistor 117 connected in series with the light emitting element 109 increases due to an increase in the ambient temperature, the conduction of the light receiving element 112 is accordingly delayed, and the relative timing of the conduction of the triac 113 and the output contact mechanism 115 may even reverse. In such a case, since the output load circuit 103 is brought into conductive state directly by the conduction of the output contact mechanism 115, electric arcs are generated at the contact points, and the basic function of the hybrid relay is totally lost.
Likewise, when the switch 105 is turned off, the timing of the operation of the photo-triac 108 may also be so unpredictable that the contact points of the output contact mechanism 115 may be disconnected after the triac 113 is brought into non-conductive state, and electric arcs may be produced in the output contact mechanism 115.
Further, the triac 113 conducts a small amount of current or leak current even when it is in its "nonconductive" state, and it is therefore preferable to disconnect the power line leading to the triac 113 to eliminate the waste of electric power and unnecessary heat generation from the triac 113 by using an auxiliary contact mechanism which may be combined with the output contact mechanism. The timing of disconnecting the power line leading to the triac must be properly arranged in relation with the connection of the output contact mechanism and the conduction of the triac so as not to disrupt the proper order of the switching actions of the output contact mechanism and the triac.
Furthermore, if the leak current is disconnected too soon after the disengagement of the output contact mechanism, electric arcs may be generated in the auxiliary contact mechanism for leak current control for the following reason. Now, a triac has the property to stay conductive once it has become conductive even after the gate voltage is reduced to "0" until the voltage across it is reduced to "0". Therefore, as shown in the graph of FIG. 6, if the timing t.sub.1 of removing the gate voltage of the triac coincides with the timing T of applying "0" voltage across it, the triac immediately turns off and no problems arises, but, if the timing t.sub.2 of removing the gate voltage falls between the adjoining timings T of applying "0" voltage across the triac, the triac stays conductive during the time interval h between the timing t.sub.2 and the subsequent timing T, and the load circuit is kept in conductive state during that time interval. Therefore, if the auxiliary contact mechanism for shutting off the leak current is disconnected during the time interval h, electric arcs are generated in the auxiliary contact mechanism for shutting off the leak current.
Obtaining an appropriate timing of such three switching actions with a sufficient accuracy have not been possible with the prior art hybrid relays. It may be conceivable to prevent the generation of electric arcs in the auxiliary contact mechanism for shutting off the leak current by adjusting the contact gap of the auxiliary contact mechanism for it, but it is difficult to achieve because it must be carried out in consideration of the gaps of other parts of the contact mechanism, and the fine adjustment of the contact gaps is technically difficult.
Furthermore, if the edges of the pulse input from the drive switch to the relay coil are rounded, it becomes even more difficult to achieve a proper timing of the above mentioned switching actions because the time point of effective energization and de-energization of the electromagnet becomes uncertain.